Write the Lewis structures for H3O+. What is the total number of available valence electrons for this ion?
The correct answer and explanation is :
Certainly! Here’s the answer and detailed explanation for the Lewis structure of H₃O⁺ (the hydronium ion):
Step 1: Determine the total number of valence electrons
- Hydrogen (H) has 1 valence electron each.
- Oxygen (O) has 6 valence electrons.
- There are 3 hydrogens: $3 \times 1 = 3$ electrons.
- Oxygen contributes 6 electrons.
- Since the species is H₃O⁺, it has a +1 charge, meaning it has lost 1 electron.
Total valence electrons = 6 (O) + 3 (H) – 1 (charge) = 8 electrons
Step 2: Draw the Lewis structure
- Place oxygen (O) in the center because it is more electronegative.
- Attach three hydrogen atoms to oxygen with single bonds.
- Each O-H bond represents 2 electrons, so 3 bonds use $3 \times 2 = 6$ electrons.
- We have 8 electrons total, so the remaining 2 electrons form a lone pair on oxygen.
Final Lewis structure:
- Oxygen has three single bonds with hydrogen.
- Oxygen has one lone pair (2 electrons).
- The positive charge (+1) indicates that oxygen has only 6 electrons around it in total (3 bonds × 2 electrons = 6 electrons), which is typical for hydronium ion.
Explanation:
The hydronium ion, H₃O⁺, forms when a water molecule (H₂O) accepts a proton (H⁺). The oxygen atom in water has 6 valence electrons, and when it forms two bonds with hydrogens, there are 2 lone pairs left on oxygen. When an extra proton attaches, it forms a third O-H bond.
Because the ion has a positive charge, it means there is one less electron compared to the neutral species. So, total valence electrons for H₃O⁺ are 8.
The oxygen atom in H₃O⁺ forms three single covalent bonds with hydrogen atoms, each sharing 2 electrons, using 6 electrons in bonds. The remaining 2 electrons exist as a lone pair on oxygen. This gives oxygen a formal charge calculation:
- Oxygen starts with 6 valence electrons.
- In H₃O⁺, oxygen “owns” 2 electrons in the lone pair plus one electron from each bond (3 bonds × 1 electron = 3), totaling 5 electrons.
- Oxygen normally has 6, but here it effectively has 5, so formal charge = 6 – 5 = +1, consistent with the ion’s positive charge.
This structure explains the hydronium ion’s trigonal pyramidal shape due to the lone pair repulsion, similar to ammonia (NH₃), but with oxygen as the central atom.